1. Field of the Invention
This invention relates to pattern detection circuitry, in particular for use in an audio signal reproduction device, and to a method of operation of such pattern detection circuitry.
2. Description of the Related Art
It is known in audio reproduction devices to use sigma-delta modulation to encode data for storage or transmission over a transmission channel. For example, in the Direct Stream Digital (DSD) data format used for encoding data for storage on Super Audio Compact Discs (SACDs), a 1-bit sigma-delta modulator is used.
In this data format, periods of silence, for example between the tracks on a CD, are represented by a repetitive 8-bit pattern with equal numbers of ones and zeros. The playback system can then detect this pattern, and can disable or mute the output stage. However, if the pattern is not successfully detected, it will produce spectral tones with large amplitudes at frequencies of one eighth of the sampling frequency, and multiples thereof.
Although these frequencies are in the inaudible ultrasonic range, they can overload the amplifier or speakers in the playback system such that they may cause audible artefacts or may even cause damage to the amplifier or speakers.